1. Field of the Invention
The present invention relates to a casting apparatus for manufacturing a metal such as a polycrystalline silicon ingot and casting method therefore.
2. Description of Related Art
It is usual to blow an inert gas so as to surround a die by an inert atmosphere for casting a raw material such as silicon (Si) or a high-melting-point-metal which tends to be oxidized when cast in a die under an ordinary atmosphere condition. In particular, it is common to blow an argon (Ar) gas to a surface of a molten silicon when a polycrystalline silicon ingot which is used for a solar cell and the like is casted.
Thus, by supplying the Ar gas, a silicon oxide (SiO) gas which is generated in the molten metal is removed and a carbon monoxide (CO) gas which is generated when the SiO gas reacts with carbon (C) which is contained in the furnace is prevented from being contained in a molten metal. In particular, it is commonly known that quite an amount of a carbon is contained in the ingot when the CO gas is contained in the molten metal; thus, the product is affected unfavorably.
In a document such as a Japanese Unexamined Patent Application, First Publication No. 2000-158096, it is disclosed that a section for supplying an Ar gas is disposed and a lid is disposed above the die so as to prevent a carbon from being contained in the ingot by restricting a CO gas in the molten metal conventionally.
However, the above conventional casting method has following problems. That is, there is a problem in that a raw raw material which is used in the casting operation is limited when the raw material is poured in the die if a lid is disposed above the die. Also, there is a concern that a molten silicon on the surface of the raw material melted while the raw material is melted is splashed and adheres to the lid; thus, an impurity is contained in the molten silicon. Furthermore, a push-out effect of an Ar gas becomes reduced when Ar gas is reduced so as to avoid a splash of molten metal which occurs when the molten metal is nearly solidified and little molten metal remains in the die. In contrast, there is a concern that the lid moves from a predetermined position because of a pressure of the gas when a greater amount of gas is blown.